Apparatus for sensing at least one parameter in water

ABSTRACT

Apparatus ( 2 ) for sensing at least one parameter in water, which apparatus comprises: (i) a conductivity sensor ( 6 ) for sensing conductivity in the water; (ii) a dissolved oxygen sensor ( 4 ) for sensing dissolved oxygen in the water; (iii) a glass substrate ( 14 ); and (iv) the conductivity sensor ( 6 ) and the dissolved oxygen sensor ( 4 ) are fabricated on the glass substrate ( 14 ) using photolithography and etching.

This invention relates to apparatus for sensing at least one parameterin water. The sensed parameter may be one or more of conductivity,temperature, and dissolved oxygen.

Apparatus for sensing conductivity, temperature, and depth is arequirement for determining the physical properties of sea water and arefrequently used to determine the practical salinity which in turn can beused to estimate density or composition. The practical salinity of oceanwater typically ranges from 33 to 37, but can be as low as 0.5 inBrackish seas. Salinity >300 is possible in landlocked water bodies.Salinity varies with both depth (non-linearly, but typically decreasesby 2 descending the first 1000 m) and location. Although someapplications need an accuracy of 0.001, a typical accuracy requirementfor satellite oceanographic applications is about 0.1, which requiresconductivity and temperature accuracies to be 0.1 mS/cm and 0.1° C.However, most oceanographic data requires an accuracy in salinity of0.01. In addition to sea water applications, sensing conductivity isalso important for freshwater. For example, river water has a normalconductivity range of 50˜1500 μS/cm; freshwater fish prefer aconductivity between 150˜500 μS/cm; typical drinking water is in therange of 50˜500 μS/cm; and industry waters can range to as high as 10mS/cm. Therefore monitoring the conductivity of freshwater is extremelyimportant for water the industry and aquaculture. Several high accuracytypes of apparatus for sensing conductivity, temperature and depth arecommercially available but they are large and power hungry, and thisprecludes their use in applications requiring miniaturisation andlong-term operation. Small data storage tags have been developed tomeasure temperature and salinity. However these do not make highaccuracy conductivity measurements.

The measurement of dissolved oxygen concentration or partial pressure inwater is required for a wide range of industrial and environmentalapplications. This includes, for example, the control of aeration insewage water treatment, assessment of eutrophication, study ofrespiration, hypoxia, and primary production in natural waters. Oxygencan be measured with Clark electrodes including adaptions for lowconcentrations and adaptions that have been microfabricated. The use ofoptical indicators known as optodes is also widespread in manyapplications. A miniaturised analytical system is known that automatesWinkler titration in situ. This requires storage of reagents andseparate systems for fluid actuation, fluid handling and opticaldetection but has good performance (precision of 0.3% relative standarddeviation). Recessed disc microelectrodes operated at steady statecurrent have also been used widely for determination of dissolved oxygenin life sciences, and for stripping voltammetry detection of heavymetals. The fabrication of micro electro-mechanical systems (MEMS)microsensors is established for recessed ring type microelectrodesoperated at steady state current.

Each of the above mentioned existing technologies has differentlimitations. Optodes suffer from a slow response time (T₉₀˜30 s) and aredifficult to manufacture in high volume and low cost. Clark electrodesare relatively complex to fabricate, are fragile, often flow sensitiveand require regular recalibration. Reagent-based systems are complexwith effects on durability and cost. Recessed bare disc microelectrodesmanufactured from gold wire or amalgam deposited into tapered glass orpolymer have poor long-term performance in natural waters unless tippedwith a membrane (which negatively impacts sensitivity, time response,and stability), and are fragile and difficult to mass-manufacture. Knownfabricated MEMS microelectrodes are needle type making them extremelyfragile. The manufacturing process for the fabricated MEMS also requiresprecision dicing, deposition and etching (including in hydrofluoricacid) in three dimensions, and is difficult to automate for massproduction.

Biofouling of sensors is a widely encountered and negatively impactslong-term performance. For example diffusion through fouled membranes isrestricted, which can affect sensitivity and time response inelectrochemical or analyte consumptive sensors. Strategies to mitigateor reduce biofouling include the use of copper, non-stick materials,mechanical wipers, biocide materials, surface texturing, naturalproducts, and artificially stimulated quorum sensing. Electric fieldsand electrochemically generated chemical environments (e.g. generationof copper and chlorine ions) have also been applied using electrodestructures incorporated into the device or monitoring system solely forthis purpose. The electrochemical generation of chlorine and hencehypochloric and hydrochloric acid in marine environments is wellestablished for biofouling reduction. To date this has been applied byuse of electrode structures external to the sensor transducer. Thetechnique has also been applied using a large scale electrode mesharound but not fabricated upon the optical window of in situ opticalsensors.

It is an aim of the present invention to alleviate at least one of theabove mentioned problems.

Accordingly, in one non-limiting embodiment of the present inventionthere is provided apparatus for sensing at least one parameter in water,which apparatus comprises:

-   -   (i) a conductivity sensor for sensing conductivity in the water;    -   (ii) a dissolved oxygen sensor for sensing dissolved oxygen in        the water;    -   (iii) a glass substrate; and    -   (iv) the conductivity sensor and the dissolved oxygen sensor are        fabricated on the glass substrate using photolithography and        etching.

The apparatus is advantageous in that the fabrication on the glasssubstrate using the photolithography and etching enables the productionat the same time of a plurality of the conductivity and dissolved oxygensensors, for example 200˜400, thereby rendering the productioncommercially feasible.

The apparatus may be one in which the conductivity sensor and thedissolved oxygen sensor are provided on opposite sides of the glasssubstrate. In this case, the conductivity sensor and the dissolvedoxygen sensor may be provided on the opposite sides of the glasssubstrate using at least one mask which is the same for both theconductivity sensor and the dissolved oxygen sensor.

The apparatus may be one in which the conductivity sensor is an opencell sensor having a physically unconstrained electric field. Knownapparatus for sensing parameters in water is such that the sensors areprovided in a tube. The tube provides boundaries and thereby causes thesensors to be closed cell sensors having a constrained electric fieldwhich is constrained by the boundaries. In contrast, the conductivitysensor and the dissolved oxygen sensor of the present invention may beopen cell sensors which do not require the electric field to beconstrained by any additional boundary or object.

The apparatus may be one in which the etching is dry etching.Alternatively the etching may be wet etching.

The apparatus may include a depth sensor for sensing the depth of thewater. The depth sensor is preferably provided on the same substrate asthe conductivity sensor and the dissolved oxygen sensor but it may beprovided on a different substrate if desired.

The apparatus may include a water barrier layer. Additionally oralternatively, the apparatus may be configured to apply a voltage signalbetween a working electrode and a reference electrode, and such that thevoltage signal provides a conditioning waveform, then a wait time thatreturns a perturbed local oxygen concentration at the working electrodeto a bulk oxygen concentration value, and then a measurement functionthat is sufficiently short that the extent of a depleted oxygen zone atthe working electrode is minimised, and hence sensitivity to flow isminimised.

The apparatus may reduce cost of manufacture through the use of planarfabrication. The apparatus may increase the frequency of validmeasurements, and may be flow insensitive and stable. The apparatus mayuse an array of bare disc platinum microelectrodes with recesses formeasuring dissolved oxygen. The principle function of each theserecesses is to reduce flow sensitivity. The microelectrode array can beinterrogated with all electrodes in parallel to increase signalmagnitude and hence performance, or addressed sequentially orindividually to further increase the frequency of valid measurementsand/or sensor lifetime. The apparatus may be able to use packagedelectronics. The apparatus may be able to use a waveform which maximisesperformance, long term stability, and flow insensitivity.

The apparatus may be able to achieve antifouling by the electrochemicalgeneration of chlorine. In addition and advantageously, the apparatusmay be able to electrochemically generate oxygen. The apparatus of thepresent invention may be able to use sensor electrodes which aresufficiently robust that they can be used as the electrodes for thegeneration of these reactive species. Hitherto known apparatus has usedelectrodes on or in the vicinity of a sensing area and that have nofunction in transduction. The apparatus may also do this, but with theadditional advantage that biofouling reduction electrodes can be createdusing the same manufacturing process as for the sensing electrodes,rather than by using additional processes as currently occurs with knownapparatus. The apparatus may use small size sensors, and theirelectrodes may cover a large area of a substrate surface. The small sizelimits the current, and hence power used when generating oxygen andchlorine. The large coverage limits the required duration of reactivespecies generation, because a high concentration can be achieved quicklyin a thin layer covering the sensor. If as in known apparatus, thereactive species generating electrodes are distant from the sensor thendiffusion and convection must transport these species to the site oftransduction. This inevitably results in a lower concentration at thesensor than at the site of generation.

Embodiments of the invention will now be described solely by way ofexample and with reference to the accompanying drawings in which:

FIG. 1 shows part of the apparatus for sensing at least two parametersin water, namely conductivity and dissolved oxygen, and alsotemperature, the apparatus comprising a sensor head and packagedelectronics, and FIG. 1 showing two sides of a chip used in the sensorhead;

FIG. 2 is a diagram of sensor chip structure with a 710 μm thick glasssubstrate, a (100˜300 nm) platinum layer, SiN and SU8 insulating layers,and a water barrier layer;

FIG. 3 shows an optical microscopy image of five 25 μm recessed bareplatinum oxygen electrodes forming an oxygen sensor array;

FIG. 4 shows one embodiment of the configuration of a sensor headcomprising a conductivity and dissolved oxygen chip, and also showingconnections to electronics apparatus;

FIG. 5 shows schematically electronics for apparatus for sensing twoparameters in water, namely conductivity and temperature;

FIG. 6 is a block diagram of apparatus comprising an electrochemicalwater-property sensor for sensing dissolved oxygen in water, theapparatus having two/three electrodes;

FIG. 7a shows four different open conductivity cells;

FIG. 7b compares the electrical field and potential for the differentcells shown in FIG. 7 a;

FIG. 7c is a table summarising resistance errors achieved by theconductivity cells shown in FIG. 7 a;

FIG. 8 show results of finite element model (FEM) of the averagetemperature change of a platinum resistance thermometer (PRT) in flowingwater, when water temperature is 1 K higher and sensor head is 6 mmhigh;

FIG. 9 shows the voltage waveform applied to a working electrode forconditioning and measurement; and

FIG. 10 shows an antifouling waveform applied to sensing electrodes.

Apparatus for Sensing Dissolved Oxygen in Water, and Optionally alsoSensing Conductivity in Water. Also Producing Apparatus For SensingDissolved Oxygen in Water by Microfabrication on a Laminar Substrate

FIG. 1 shows apparatus 2 for sensing at least one parameter in water.More specifically, FIG. 1 shows apparatus 2 for sensing dissolved oxygenin water. The apparatus 2 comprises a dissolved oxygen sensor 4.

The apparatus 2 shown in FIG. 1 is such that it is also able to sense asecond parameter, namely conductivity in the water. The apparatus 2 alsocomprises a conductivity sensor 6.

The apparatus 2 shown in FIG. 1 is also such that the dissolved oxygensensor 4 is able to be produced by fabrication on a laminar substrate inthe form of a chip. The fabrication may be regarded as microfabrication.

The apparatus. 2 shown in FIG. 1 comprises a sensor head which may beused together with suitably packaged electronics. FIG. 1 shows two sides8, 10 of a chip used in the sensor head. The left side 8 of FIG. 1 showsa layout for sensing the conductivity, and the right side 10 of FIG. 1shows a layout for sensing the dissolved oxygen.

FIG. 2 is an illustrative schematic of a cross section of the chip shownin FIG. 1 and in the region of the microelectrodes. To fabricate thechip, electrodes were deposited onto 710 μm thick 6″ diameter glasswafers by sputtering Ti and Pt. The microelectrodes were then shaped byconventional photolithography and ion beam milling. A 300 nm layer ofSiN was then deposited over the entire wafer; followed by 25 μm layer ofan epoxy-based negative photoresist known as SU8. The SU8 was exposedand developed through a photomask to open windows in specific locationson the central working electrode for the dissolved oxygen sensor, andthe conductivity electrodes. The SiN in these exposed areas was then dryetched to expose bare platinum, thus forming exposed conductivityelectrodes 12 and an array of recessed microelectrodes 14 for oxygensensing, as shown in FIG. 2 and FIG. 3. In order to insulate the SU8from water uptake, a hydrophobic polymer was deposited and patterned toexpose platinum for the conductivity and dissolved oxygen sensors.Individual chips were cut from the 6″ wafer by scribing and dicing.

In the embodiment shown in FIG. 1, the apparatus 2 with the dissolvedoxygen sensor 4 and the conductivity sensor 6 comprises a glasssubstrate onto which are microfabricated Pt conductivity electrodes 12for conductivity and temperature measurement, the later based on a PRT,and an array of exposed Pt microdisc microelectrodes 14 for thedissolved oxygen sensor 4. The conductivity sensor 6 consists of fourconcentric ring electrodes 12 (black in image) made from 100˜300 nmthick Pt.

The PRT is made from a continuous line of 20 μm wide Pt thin filmelectrodes. The chips are double sided with exactly the same design forthe conductivity and temperature sensors on the reverse side and aremanufactured using double-sided lithography. Strain gauge effects canoccur if any stress is applied to the glass substrate, bending thechips, stretching or compressing the Pt wires, and changing the PRTresistance. Using double-sided chips eliminates this effect. If one sideof the PRT is stretched, increasing the resistance, the other side ofthe PRT will be compressed, decreasing the resistance, therebycompensating for the effect. Therefore this configuration eliminatesstrain gauge effects in the PRT. Strain gauge effects can be induced bywater flow or the stress of the laminar material of the sensor chip.Water absorption into the laminar material, especially the SUB, canchange the stress and strain gauge effects, introducing drift to thetemperature sensor. Without the water barrier layer, a single-sided PRTcould drift by as much as 0.1° C. due to water absorption into the SU8.This drift can be markedly reduced using the double-sided structure. Itenables the sensor to be mounted vertically out form a surface. This‘fin’ arrangement improves flushing and thermal contact with the water,whilst minimising thermal contact to the support/housing and henceimproves the time response of the conductivity and temperature sensors.The conductivity sensor operates in the conventional four-electrodeconfiguration with current injection from the outer electrodes andvoltage measurement from the inner pair of electrodes.

Temperature sensing is performed on both sides of the chip, whereasconductivity is measured on the opposite side 8 to the oxygen sensorside 10. The oxygen sensor array can be formed on any platinum electrodeat any location on the chip, but, as shown in FIG. 1, is preferablyfabricated on the central spot electrode on one side 10 of the chip.Operation in standard two and three electrode electrochemical sensing ispossible using ring electrodes not used for conductivity see FIG. 4. Ifsensing conductivity on one side of the double sided chip is required,then other ring electrodes can also be used. Alternatively, off chipauxiliary and reference electrodes can be used (e.g. bare wire, or gelencapsulated reference, e.g. formed from silver for a Ag/AgClreference).

Water conductivity and oxygen diffusion coefficient is a function oftemperature. As water temperature varies with location, errors can beintroduced in the salinity and oxygen calculation if the watertemperature around the temperature sensor is different from that aroundthe conductivity and oxygen sensor. Advantageously, all three sensorsmay be integrated close together on a single substrate, minimising thistemperature error.

An impedance measurement circuit 10 to support the conductivity andtemperature sensor is shown in FIG. 5. The circuit 10 is divided into ananalogue part 18 and a digital part 20. In the analogue part 18, a 16bit/100 ksps digital-to-analogue converter (DAC) and a low pass filterLPF1 generate a sine-wave voltage signal at 1.56 kHz (100 kHz/64). Atrans-impedance amplifier (I/V) is used to convert the voltage signalinto current for the conductivity and temperature sensor block CIT. Aninstrumentation amplifier IA with high input impedance and differentialinterface is used to amplify the voltage response from the C/T sensorblock. After removing high frequency noise using a low pass filter(LPF2), the amplified voltage response is converted into a digitalsignal by a 16 bit/100 ksps analogue-to-digital converter (ADC). Theinjection current (I) and gain of the instrumentation amplifier (G) arecontrolled, by multiplexing the feedback resistors of thetrans-impedance amplifier and the instrumentation amplifierrespectively.

Both the gain of the amplifiers and the value of the feedback resistorsdrift with temperature and time, affecting G and I. To minimize thiserror, two calibration resistors with extremely low temperaturecoefficient (0.6 ppm/° C.) and excellent load life stability (0.005%drift after 2000 hours) are embedded in the conductivity and temperaturesensor block. A 4-way multiplexer is used′ to select one of theconductivity and temperature sensors or the calibration resistors, andthe drift error only depends on the stability of the calibrationresistor. As the environment temperature varies from 0 to 30° C., theseonly drift by up to 0.0018%. Typically the apparatus takes 100 ms for asingle channel measurement with typically a 10 second measurementinterval. With this 1% load rate, the calibration resistor only driftsfor 0.0002% per year and the system has a theoretical annual 0.002%drift. The calibration resistors provide a reference phase of 0° whichallows the phase of the impedance to be determined by comparing thephase of the digital sine-wave from the sensor with that from thecalibration resistors.

The digital part 20 of the circuit 10 performs digital signalprocessing, data storage, communication with a personal computer, andpower management (including wake up). A low power field-programmablegate array (FPGA) (Actel IGLOO AGL600V2) with a system-on-a-chip (SOC)solution and a 64 MB flash memory are used in the digital circuit. Asshown in FIG. 5, four modules are implemented in the FPGA, including amicrocontroller unit module (MCU), a digital signal processing module(DSP), a waveform module, and a power control module. The waveformmodule is used to control the digital to analogue converter, andgenerates the sine-wave signal. The DSP is used to calculate theamplitude and phase of the digital sine-wave sampled by the analogue todigital converter (ADC). The power control module is used to control thepower (on/off) of the analogue circuit, and the wake-up/sleep mode ofthe digital circuit. The MCU is used to configure the other modules,collect the processed data from the DSP, store data into the externalflash memory, and communicate with the PC through a universal serial bus(USB) interface. An advantage of using a digital and analogue converterrather than a signal generator to generate the sine-wave is that theratio between the sine-wave and digital to analogue converter samplingfrequency is accurately known. Therefore a 3-parameter sine-fittingalgorithm is implemented in the DSP. If this ratio were not known, thefrequency would also need to be determined and such a 4-parameteralgorithm would be unsuitable for real-time processing.

In a different design, the MCU is an external PIC18LF26J11micro-controller, and the FPGA is an Actel IGLOO Nano AGLN250V2 toreduce the printed circuit board (PCB) dimension and power consumption

The sine-fitting algorithm is similar to DC average but for AC signalwith a certain frequency. Therefore the measurement noise can be reducedby increasing the sampling length and time, except the quantizationnoise of the DAC and ADC. A way to minimise noise is to arrange themeasurement noise to be the same level as the 16-bit quantization noise(0.002% of the full scale). Typical measurement times for this conditionare 100 ms; 20 ms for signal set-up and 80 ms for measurement. Thecircuit has an accuracy of 0.002% in amplitude and 0.02° in phase, whichtranslates into 1 μS/cm accuracy for conductivity in the range 10˜50mS/cm, and 0.0006° accuracy for temperature in the range 0˜30° C.

FIG. 6 shows a schematic of the electronics 22 used to control andinterrogate the dissolved oxygen sensor 4. Most of the electronics 22are identical to the electronics for the conductivity and temperaturesensors shown in FIG. 5, but they differ in the analogue circuit toenable either conventional two, or three electrode electrochemistry. Thetime response of the analogue circuit is sufficient to enable operationat high frequency (>500 kHz bandwidth). Both circuits may be implementedon a single printed circuit board or as an application specificintegrated circuit.

Analogue switches are also included such that each of the electrodes canbe connected to a low impedance amplifier with its output voltage set bythe digital controller. This enables a digitally controlled voltage tobe applied to the sensor electrodes. This is used to apply theappropriate waveforms for biofouling prevention by the electrochemicalgeneration of chlorine (and hence hypochloric and hydrochloric acid) andif required oxygen. The electrochemical cells are formed between theplatinum sensing electrodes and the Ag/AgCl or on-chip referenceelectrode.

Apparatus for Sensing Conductivity in Water

Referring now to FIGS. 7a and 7b , there is shown part of apparatus forsensing at least one parameter in water. More specifically, the sensedparameter is conductivity and the sensing is effected by a conductivitysensor. The sensing is effected by the use of four electrodes and theconductivity sensor is an open cell sensor which has a loop design andwhich has been microfabricated.

Proximity Effects

The electric field generated by the electrodes is distributed over thevolume around the sensors. Therefore the cell constant will be modifiedif any insulating or conducting object moves into the electric field;this is the proximity effect. Proximity effects are commonly reduced byconstraining the electric field inside a channel. However the use of achannel can be problematic since a continuous recirculation of the watermust be ensured. The planar four electrode device has a very smallleakage of field and this is shown in FIGS. 7a and 7b where fourdifferent designs were evaluated.

FIG. 7a shows diagrams of four different geometries of open conductivitycells. Cell A has two electrodes on the same side. Cell B has twoelectrodes on the opposite sides of the substrate. Cell C has threeelectrodes on the same side of the substrate. Cell D is a roundelectrode with a ring electrode on the same side of the substrate.

FIG. 7b shows the electric potential distributions for the fourdifferent open conductivity cells as determined by FEM simulation.

FIG. 7b also shows that, when the cell is immersed in seawater, a partof the current leaks into the surrounding water. If an object moves nearthe electrodes, this external current path is modified.

In a simulation, the four different cells were modelled as insulatingcubes all of the same dimension (10×10×0.5 mm). The surrounding waterwas modelled as a sphere with a variable diameter from 10 mm to 200 mm.By comparing changes in the cell resistance as a function of thediameter of this sphere of water, the performance of the system could beestimated in terms of sensitivity to proximity effects. Less resistancechange for a given change in the diameter of the water sphere means thatthe system has smaller proximity effects, and has better performance.The FEM simulation results are also affected by the mesh density. Tominimise this, the water sphere was not placed directly around the cell.Instead, the surrounding medium was divided into several partsconsisting of concentric spherical shells with diameters of 10, 20, 40,100, and 200 mm. To simulate a water sphere of a given diameter, forexample 20 mm, the medium inside the 20 mm spherical shell was modelledas water with conductivity of 60 mS/cm, while the medium outside was setto insulator. Therefore, for one cell, all simulations shared the samemesh, minimising the mesh error effect. In these simulations, mediaoutside the certain diameter is set to be an insulator. By setting thismedia as a conductor, it gave the opposite effect (the same amount).

Referring to FIG. 7c , the resistance errors summarised in Table 1 showthat cell D of FIGS. 7a and 7b has the best performance. This is becausecell D confines the electrical potential within a small symmetricvolume. When the dimension of the surrounding water is bigger than 40mm, four times the characteristic cell dimension, the resistance of cellD is almost unchanged within a tolerance of 0.01%. This means that cellD is up to 99.99% accurate if an insulating or conducting object is 20mm or more away from the centre of the chip. Furthermore, even if thediameter of the surrounding water is only 10 mm, which just covers theelectrodes, the error is only 1.42%. Cell C (an axial symmetric design)is the second best, with error approximately twice that of cell D. CellA and cell B have much greater proximity effects. Cell B with electrodeson the opposite sides of the substrate, does not work if the watersphere is smaller than the substrate. However, if the water sphere isbigger than the substrate, the performance of cell B is slightly betterthan cell A.

Apparatus for Sensing Temperature in Water

Referring again to FIG. 1, there is shown apparatus 2 for sensing atleast one parameter in water. The sensed parameter is temperature andthe temperature is sensed by a temperature sensor 24. The apparatus 2includes strain relief/compensation using resistance pairs on both sidesof a substrate monolith, i.e. the glass substrate 14. The temperaturesensor 24 is described in more detail as follows.

Platinum Resistance Thermometer (PRT)-Bridge Temperature Sensor 24

PRTs have a higher measurement range and stability than thermocouples orthermistors, and are ideally suited for precision applications. However,the sensitivity of a PRT is relatively low. To enhance the sensitivity abridge circuit was made, consisting of two PRTs and two precisionresistors. Each platinum resistor is fabricated on each side of theglass substrate from a 20 μm wide and about 24 cm long platinum in asnake, giving a PRT with a resistance of 6.4 kohms at 20° C. Two 6.4kohms precision resistors are soldered between the two PRT resistors tocreate a bridge circuit. Thin film PRTs are relatively low cost and havea fast response, although the different thermal expansion rates for theglass substrate and platinum might cause strain gauge effects, but thisis hard to estimate. However, as glass is a solid material whileplatinum is extremely thin, the thermal expansion of the glass willdominate, and this will be proportional to temperature. The common modestrain gauge effect can therefore be ignored with little penalty,especially if the sensor is calibrated. However, strain gauge effectsmay still be observed if the source of strain is mechanical anddifferential (e.g. bending). To circumvent this problem, the PRT sensoris duplicated on the back side of the glass substrate 14, and the deviceis operated in differential mode to effectively eliminate the straingauge error. This arrangement forms strain compensation means. Toenhance the sensitivity of the PRT, an electric bridge consisting of twoPRT resistors and two precision resistors is used as the temperaturesensor 24.

Response Time

The response time of the temperature sensor 24 not only depends on thesensor chip, but also depends on the package thermal mass and water flowrate. To simulate the behaviour of the system, a FEM model was used toplace a virtual sphere of water, 50 cm diameter at a temperature of 290Karound the sensor. The boundary condition of the water at the surfacewas set to 290K and the initial temperature of the sensor head set to289K, 1K below the surrounding water. The response time in static watercan be determined by analysing how the temperature of the PRT changeswith time. However, whilst the surrounding water heats the sensorpackage, it is also cooled by the sensor package at the same time. Atthe very beginning, both sensor head and the package are heated by thewater, but the temperature of the sensor head grows faster, due to thehigher thermal conductance of the glass, its lower thermal mass and therelativity larger contact area. At longer times, because of the bulk ofthe housing, together with its lower temperature, the surrounding wateris cooled below the sensor head, further cooling the sensor. In staticwater, the cooled water surrounding the package is not refreshed bywater with a higher temperature and the cooled surrounding water onlyreceives heat by thermal conduction, which takes a long time because ofthe relatively low thermal conductance of the water. In flowing water,cooled surrounding water is heated by convection, so that thesurrounding water temperature remains static. To simulate thissituation, the temperature of the boundary is set to a constant 290K,and the sensor head is placed at a distance of 6 mm from the housing.The results are shown in FIG. 8. The sensor head takes 0.5 seconds toreach 85% change, and 2 seconds for a 99% change. Furthermore, nocooling occurs.

Oxygen Sensing

Bare disc microelectrodes are simple to manufacture, but suffer fromflow sensitivity and sensitivity to changes in complex chemical media.To address these known challenges the sensor was recessed in a pit, andoperated with a novel waveform that maximises sensitivity to oxygen,provides electrode conditioning to maintain performance, and shortensthe measurement period. Previous recessed electrodes have been operatedwith sensing of the diffusion limited (steady stage current). Incontrast, the apparatus of the present invention may be operated with ashort measurement period compared to a longer wait (prior tomeasurement) and electrode conditioning cycle (after each measurement).This has a number of advantages as follows.

-   1. The use of a conditioning cycle maintains the condition of the    electrode surface even in complex median (such as seawater)    enhancing long term performance.-   2. The use of a recess, for example a pit, in conjunction with a    short measurement period reduces the volume of the area of depleted    oxygen (the diffusion bubble) such that it remains within the    recess, and this dramatically reduces flow sensitivity even at low    aspect ratios.-   3. The need for a large aspect ratio recess or recesses, or a    membrane of reduced oxygen transmissibility, to reduce flow    sensitivity is obviated. The use of low aspect ratio recesses has    the advantage that it eases fabrication, increases sensitivity to    oxygen, and improves time response. Flow insensitivity, and the    duration of the measurement current may be improved with increased    recess depth and aspect ratio, but this increases the time for    oxygen to diffuse from the bulk into the recess, and therefore would    decrease sensitivity whilst increasing the minimum time between    valid measurements.

The design of the apparatus may be a compromise between flowinsensitivity on the one hand and sensitivity and frequency ofmeasurement on the other. In the current embodiment, the microelectrodeis at the bottom of a recess of 25 μm depth and 25 μm diameter.

Apparatus Comprising a Dissolved Oxygen Sensor with ConditioningWaveform and Delay

There is now described apparatus for sensing at least one parameter inwater. The sensed parameter is dissolved oxygen and the dissolved oxygenis sensed by a dissolved oxygen sensor which has an electrode. Theapparatus is electrode driven by a conditioning waveform and then adelay. More specifically, the electrode is driven by drive means whichprovides a conditioning waveform, then a wait time, and then ameasurement function. The apparatus is advantageous in that theconditioning waveform is able to ensure that the apparatus operates withcorrect voltages, frequencies etc. An example of the conditioningwaveform for the dissolved oxygen measurement is shown in FIG. 9. Theconditioning waveform enables the electrode to sense the dissolvedoxygen. The conditioning waveform and the wait time work together toensure the precise and accurate functioning of the apparatus.

The apparatus may be one in which the conditioning waveform duration is100 ms-800 ms.

The apparatus may be one in which the post conditioning waveform delayis 200 ms-1800 ms.

An important advance is that the electrode is returned to a repeatableand stable state by periodic electrode conditioning, but following thisa wait period is used to return the perturbed (by conditioning) localoxygen concentration in the recess, for example the pit, to the bulkvalue. Then a short measurement period is used that is sufficientlyshort that the depleted oxygen zone (because of the reductivemeasurement) does not extend beyond the pit or boundary layer above therecess and hence sensitivity to flow is minimised. This is because thedepleted zone does not enter the region where convective mass transferoccurs. This procedure is applied cyclically. However it should be notedthat this cyclic stepped chronoamperometry is not the same as cyclicvoltametry as used in other electrochemical sensor systems. Themeasurement (of current which is proportional to oxygen concentration)is made at a fixed voltage for short duration whereas in cyclicvoltametry current is measured at a range of voltages (as voltage isswept or stepped) and these results combined or processed to calculatedissolved oxygen concentration.

Even with the waveform described above, there is some residual drift inthe measurement of dissolved oxygen with time and electrode condition.To counter this, two further approaches are possible.

-   1. The voltage used during the measurement function may be varied    over −0.019 to −0.219 V (vs the water, as the reduction voltage    shown as −0.069 V in FIG. 9) and the current at the working    electrode recorded at discrete voltages across this range. These    current values change with time and electrode condition, but are    fitted to a polynomial to give a single value that is invariant with    time and electrode condition. The coefficients for the polynomial    are generated through calibration at a known temperature and    dissolved oxygen concentration and are subsequently applied during    measurements. This calibration could be performed in situ in regions    of known and stable oxygen concentration, such as the deep sea.-   2. Periodically, the measurement voltage used in the waveform can be    stepped over a wider range of −0.669 to 1.131 V (vs water) to    generate a cyclic voltamogramme (CV). The application of this    potential provides a more aggressive electrode conditioning,    resetting its condition to a stable state. Following a CV, the    electrode calibration returns in an approximately exponential (in    time) manner (over a few hours) to a stable value. This exponential    (in time) is repeatable and is characterised. Data processing is    then used to remove this signal from the measurement.

The conditioning waveform duration is ideally 100 ms to 800 ms(optimised to achieve sensitivity and stability of the sensor). The postconditioning-waveform delay is ideally 200 to 1800 ms (and is a functionof recess depth and tolerable flow sensitivity). Longer delays reduceflow sensitivity but reduce sample frequency. Deeper recesses requirelonger delay, reducing the sample frequency.

A smaller diameter and shallower recess would enable a shorter postconditioning delay, but this would reduce the sensing current and,measurement duration. A shallower recess without changing the diameterwould enable a shorter post conditioning delay, but this would increasethe flow sensitivity. Using delays shorter than the re-equilibrationtime of the recess is undesirable and causes instability and or flowsensitivity.

Longer delays enable effective measurements but limit the frequency ofvalid measurements and reduce signal to noise ratio. Thus for example,if a measurement is required only once an hour then a delay of severalminutes might be acceptable. However, to achieve maximum measurementfrequency (minimum measurement period) the delay should be limited tojust longer than the re-equilibration time.

The duration of the conditioning waveform may be shortened inenvironments where less electrode degradation occurs for example in purelaboratory media a duration of 50 ms per cycle may be sufficient. Longerdurations could also be used more infrequently (e.g. conditioning every5 seconds) but longer periods between conditioning can result in driftand loss of electrode condition. Too much cleaning results inaccelerated electrode erosion (limiting lifetime) and rougheningresulting in a change in the sensitivity of the sensor and hence drift.Therefore whilst the sensor, or similar sensors, may be operated outsidethe aforementioned ranges for high frequency continuous measurements inseawater these ranges result in optimal performance.

Apparatus Comprising an Electrochemical Water Property Sensor and Usinga Biofouling Mitigation Waveform

Referring to FIG. 10, there is shown an example of an antifoulingwaveform applied to sensing electrodes in apparatus for sensing at leastone parameter in water. The parameter is sensed by at least oneelectrochemical water-property sensor. The sensor is such that it cleansitself by liberating chlorine. In addition the sensor couldadvantageously also generate oxygen.

The use of analogue switches, a low impedance amplifier, and digitalcontrol allow a wide range of voltage (and hence current) waveforms tobe applied to sensing and/or antibiofouling specific electrodes on or inthe vicinity of the sensor, against an Ag/AgCl reference electrode. Theelectrochemical generation of chlorine is particularly effective, butraising the oxygen concentration can also have advantages. Electricfields may play a role in fouling prevention/mitigation, as canalternating current.

This enables a wide range of waveforms to achieve fouling reduction.However, it may be possible to optimize the effectiveness of thewaveform for particular environmental conditions. There is a trade-offbetween the potential and duration of applied voltage and powerconsumption of this fouling mitigation. Therefore waveforms should beoptimized for both effectiveness and power consumption. As mentionedabove, FIG. 10 shows an antifouling waveform applied to sensingelectrodes. Chlorine generation is achieved by applying 2.24 V to theAgCl reference electrode driving the working electrode to >−2.56 V.Chlorine is generated from chloride at −1.36 V (vs a standard hydrogenelectrode) and thus this overpotential ensures chlorine is produced. Inone preferable embodiment, the current is limited by inclusion of aresistor between applied voltage and the electrode. This suppresses theelectrochemical dissolution of the working electrode and increases thelife of the sensor. Periodically the polarity of the applied voltage isreversed and −1.36 V applied to the reference electrode. This isprimarily used to regenerate the Ag/AgCl electrode (this reaction occursat 0.222 V vs a standard hydrogen electrode) and hence as long anysuitable overvoltage is used then this is achieved. This means that thereference electrode is not consumed. During this reference electroderegeneration step, the working electrode is driven positive into thehydrogen generating region. A resting potential in the region of 0.44 Vis used.

FIGS. 1-10 show apparatus for sensing a plurality of parameters inwater, with different parts of the apparatus being able to sensedifferent parameters. The different parts of the apparatus are all ableto be provided in a single piece of apparatus. The single piece ofapparatus may act as a miniature high precision conductivity,temperature and dissolved oxygen sensor. The apparatus may be of specialuse for ocean monitoring. The apparatus is manufactured usingmicrofabrication technology. One embodiment of the apparatus may be madefrom platinum, patterned on a glass substrate 14. A four-electrode ringconductivity sensor may be combined with a platinum resistor temperaturebridge to produce an integrated conductivity and temperature sensor. Adissolved oxygen sensor 4 (with a second temperature sensor) may beintegrated onto the reverse side of the chip as shown in FIG. 1. Ageneric impedance measurement circuit may be used with a 3-parametersine fitting algorithm. Conductivity and temperature accuracies may bebetter than ±0.01 mS/cm and ±0.005° C. respectively. The dissolvedoxygen resolution may be better than 3 μm. Long-term operation innatural environments may be enhanced by the use of electrochemicalgeneration of chlorine on the sensor electrodes themselves, whichreduces the onset and affect of biofouling.

Individual components shown in the drawings are not limited to use intheir drawings and they may be used in other drawings and in all aspectsof the invention.

The invention claimed is:
 1. Apparatus for sensing at least oneparameter in water, which apparatus comprises: (i) a conductivity sensorfor sensing conductivity in the water; (ii) a dissolved oxygen sensorfor sensing dissolved oxygen in the water; (iii) a glass substrate; (iv)the conductivity sensor and the dissolved oxygen sensor are fabricatedon the glass substrate using photolithography and etching; and (v) theapparatus includes a digitally controllable electronic circuit thatapplies a voltage signal between a working electrode and a referenceelectrode, and such that the voltage signal provides a conditioningwaveform, then a wait time that returns a local oxygen concentration atthe working electrode to a bulk oxygen concentration value, and then ameasurement function that controls the extent of a depleted oxygen zoneat the working electrode, and hence sensitivity to flow.
 2. Apparatusaccording to claim 1 in which the conductivity sensor is an open cellsensor having a physically unconstrained electric field.
 3. Apparatusaccording to claim 1 in which the etching is dry etching.
 4. Apparatusaccording to claim 1 in which the etching is wet etching.
 5. Apparatusaccording to claim 1 and including a depth sensor for sensing the depthof the water.
 6. Apparatus for sensing at least one parameter in water,which apparatus comprises: (i) a conductivity sensor for sensingconductivity in the water; (ii) a dissolved oxygen sensor for sensingdissolved oxygen in the water; (iii) a glass substrate; (iv) theconductivity sensor and the dissolved oxygen sensor are fabricated onthe glass substrate using photolithography and etching; (v) anelectrical insulation layer which covers electrically conducting partsof the apparatus that require protection from the water, and which has aboundary which defines the geometry of exposed metal that defines theconductivity sensor and the dissolved oxygen sensor; (vi) a waterbarrier layer which covers the electrical insulation layer, whichprotects the electrical insulation layer from the water, and which has aboundary which defines the geometry of the exposed metal that definesthe conductivity sensor and the dissolved oxygen sensor, and (vii) theapparatus includes a digitally controllable electronic circuit thatapplies a voltage signal between a working electrode and a referenceelectrode, and such that the voltage signal provides a conditioningwaveform, then a wait time that returns a local oxygen concentration atthe working electrode to a bulk oxygen concentration value, and then ameasurement function that controls the extent of a depleted oxygen zoneat the working electrode, and hence sensitivity to flow.
 7. Apparatusfor sensing at least one parameter in water, which apparatus comprises:(i) a conductivity sensor for sensing conductivity in the water; (ii) adissolved oxygen sensor for sensing dissolved oxygen in the water; (iii)a temperature sensor for sensing the temperature of the water; (iv) aglass substrate; (v) the conductivity sensor, the dissolved oxygensensor and the temperature sensor are fabricated on the glass substrateusing photolithography and etching; (vi) the conductivity sensor and thedissolved oxygen sensor are provided on opposite sides of the glasssubstrate, (vii) the conductivity sensor and the dissolved oxygen sensorare provided on the opposite sides of the glass substrate using at leastone mask which is the same for both the conductivity sensor and thedissolved oxygen sensor, whereby the conductivity sensor and thedissolved oxygen sensor have the same metal layer design on both sidesof the glass substrate; (viii) an electrical insulation layer whichcovers electrically conducting parts of the apparatus that requireprotection from the water, and which has a boundary which defines thegeometry of exposed metal that defines the conductivity sensor and thedissolved oxygen sensor; (ix) a water barrier layer which covers theelectrical insulation layer, which protects the electrical insulationlayer from the water, and which has a boundary which defines thegeometry of the exposed metal that defines the conductivity sensor andthe dissolved oxygen sensor; and (x) the apparatus includes a digitallycontrollable electronic circuit that applies a voltage signal between aworking electrode and a reference electrode, and such that the voltagesignal provides a conditioning waveform, then a wait time that returns aperturbed local oxygen concentration at the working electrode to a bulkoxygen concentration value, and then a measurement function thatcontrols the extent of a depleted oxygen zone at the working electrode,and hence sensitivity to flow.